The objective of this project is to determine the magnitude of the role of interstitial adenosine in producing the coronary vasodilation associated with catecholamine infusion, cardiac pacing and hypoperfusion. To do this we need to increase our understanding of the role of endothelial cells in capillary exchange of adenosine. Our hypothesis for capillary exchange of adenosine includes diffusion through water filled channels as well as endothelial cell uptake and release of adenosine. The tests of the hypothesis will consist of single pass indicator diffusion experiments on isolated guinea pig hearts involving the simultaneous injection of three tracers: 125I-albumin (RISA), 3H-arabinofuranosyl hypoxanthine (3H-araH) and 14C-adenosine (14C-ADO). A newly developed mathematical model will aid in the analysis of these data. RISA allows optimizationof model parameters associated with intravascular transit and volume. We will use H-araH, an analogue of adenosine which is not transported by the nucleoside carrier, to optimize model parameters related to extracellular diffusion of adenosine and interstitial volume. Finally, 14C-adenosine will be used to characterize its carrier mediated transport and subsequent metabolism by endothelial cells. Further information about the symmetry of the carrier on thelumenal and ablumenal surface of endothelial cells will be obtained in parallel experiments on C-5 feet-deoxyadenosine which is transported by the carrier, but not metabolized. Dipyridamole and competing concentrations of unlabelled adenosine will be used to further characterize the role of carrier mediated transport by endothelial cells. We will also selectively label the adenine nucleotide pool of endothelial cells in order to determine the conditions under which adenosine is released from in situ endothelium. Once a model of the capillary barrier has been established, we will use it to calculate [ADO]ISF in a variety of conditions including catecholamine infusion alone, and in the presence of theophylline or dipyridamole, pacing and hypoperfusion. These experiments are designed to further test the role of adenosine in the local regulation of coronary blood flow flow insituations in which there are observations which are inconsistent with the adenosine hypothesis. The experiments will also provide new information on the role of endothelial cells in nucleoside homeostasis in the heart as well as a new approach to the investigation of the metabolism of in vivo endothelial cells in the heart.